Late period...confused.

Nicole
Glow told me my period was due today. Nothing yet. It's usually here first thing in the morning when I get it. Something I usually wake up to. Yay lol 
When I told the app it was late it changed its prediction for 2 days from now. My cycle is regular 28 days usually. When do I count my next period from? The last day of my last one?
I tested up until 2 days ago & negative. But now wondering if I should test again. I have clear CM today a lot.